Image formation is carried out by a series of processes of electric charging, exposure, development, transfer and cleaning on the surface of an electrophotographic photoreceptor, hereinafter also simply referred to as photoreceptor, used for the electrophotographic image forming method. Recently, organic photoreceptor using an organic compound is widely applied as the electrophotographic photoreceptor. As to the organic photoreceptor, influence to the environment of it is low and ones capable of corresponding to various light wavelengths of light are easily developed.
At early time, the organic photoreceptor had problem of durability versus mechanical external force or chemical action. In concrete, occurrence of wearing or scratches by friction with the charging means, developing means, transferring means or cleaning means, and surface degradation caused by active oxygen such as ozone or nitrogen oxide formed on the occasion of the corona charging were tend to occur.
Many trials have been carried out for improving the durability of the surface of organic photoreceptor. As an example, it was tried to make the charge transfer layer to double layer for raising the physical strength of the photoreceptor surface or to add silica particles in the outermost layer for raising the mechanical strength of the photoreceptor surface, cf. Patent Documents 1 and 2 for example. As results of such the efforts of the investigation, the durability of the organic photoreceptor has been considerably improved recently.
It is usual method for improving the durability to raise the anti-wearing property by raising the hardness of the surface. However, a problem is caused that the surface property of the photoreceptor tends to be degraded, tar from improving, by such the method. It is made clear that adhesion of the toner and reaction products formed by chemical reaction with ozone onto the photoreceptor surface is caused accompanied with the image formation and such the foreign materials should be effectively removed for keeping the stable image quality. On the usual photoreceptor having low anti-wearing ability, the foreign materials can be removed together with wearing of the photoreceptor surface, but the foreign materials are difficultly removed from the photoreceptor surface which is difficultly worn. As a result of that the degradation of the surface property of photoreceptor is caused.
Recently, image forming apparatuses using the electrophotographic technology are improved from various viewpoints so that high quality printed images can be obtained at high speed. Therefore, the image forming apparatuses utilizing electrophotography are begun to be applied in the field of short-run printing.
In the field of short-run printing, different from in the office-use, character images are often printed on a solid image accompanied with the consumption of a large amount of toner, for example, yellow letter images printed on a blue background.
A problem of occurrence of raindrop-like image defects (stains) on the printed image is caused when the images with the consumption of a large amount of toner such as solid images are continuously printed by the above image forming apparatus.
The occurrence of raindrop-like image defect is a phenomenon that toner fine particles and the external additives of the toner, which are rightfully removed by the cleaning device, slip through the cleaning device and adhere onto the photoreceptor surface in a form of lump having a width of from 2 to 200 μm and a length of from 10 μm to 2 cm and adhering portion is appeared as the raindrop-like defect on the printed image.
For raising the cleaning ability, an image forming apparatus in which a means for applying a lubricant onto the photoreceptor surface has been proposed, cf. Patent Document 3 for example.    Patent Document 1: JP A H02-160247    Patent Document 2: JP A H07-333881    Patent Document 3: JP A 2003-58009